The present invention relates to an apparatus for producing deposits by epitaxy on a substrate using molecular jets and a compound, which is normally liquid at ambient temperature.
In the vacuum deposition of a product by epitaxy on a substrate to be covered, use is made in known manner and as shown in FIG. 1 of an ultravacuum enclosure 1, which is e.g. kept at a pressure of approximately 10.sup.-9 Torr and cooled by a heat shield, which has a peripheral layer of liquefied gas at low temperature, e.g. nitrogen. FIG. 1 shows that the cell 3 for the production of the actual molecular jet used for the epitaxial deposit and containing the products 4 to be deposited, maintained at the necessary temperature by electric furnace 5. A concealable screen 6 can be rotated by an external control means 7 and can consequently release the molecular jet formed by the action of furnace 5 in enclosure 1, so that it can reach the substrate 8 to be covered.
Installations of this type lead to operating problems, which have not hitherto been satisfactorily solved when one of the products to be evaporated on the substrate in the form of a molecular jet is a normally liquid substance having a high vapour pressure at ambient temperature. A typical example of this hypothesis is the epitaxial growth of layers of mercury compounds projected onto the substrate by a molecular jet using the apparatus of FIG. 1. In evaporation cells, which use a substance in the liquid state for carrying out molecular jet epitaxy, two essential problems have to be solved:
(a) the high vapour pressure at the ambient temperature of the material to be projected or sprayed (e.g. mercury or an organoleptic compound) is incompatible with a prolonged residence in the ultravacuum environment of the working enclosure;
(b) the high consumption of the liquid product during the epitaxy operation and e.g. in the case of mercury to be sprayed in molecular jet form, the consumption can be roughly 20 g/h.
Attempts have already been made to solve the above problems in industrial installations by using a retractable cell for producing the molecular jet, it being possible to separate said cell from the main enclosure when it is not in use. However, this solution is difficult to realize, because it must be removed from the enclosure, which requires relatively complex technology.
Consideration has also been given to cooling the material to be projected at very low temperatures, in order to reduce its vapour pressure when the cell is not in use.
The problem of the consumption has been compensated in existing systems by discontinuous cell filling methods, which do not require the reventing of the cell.
Unfortunately, all the installations putting the above means into use are very complex, sometimes difficult to use and often costly. Moreover, all the sources used for producing molecular jets of normally liquid substances at ambient temperature, lead to charge variations in the cell during use, such as e.g. during the formation of a crystalline Cd Hg Te layer, so that it is easy to empty more than half the cell. This variation leads to a poor temperature regulation of the cell due to the modification of its charge, as well as to a significant change to the molecular jet emission or discharge profile during use, due to the liquid level in the cell dropping, so that the angular dispersion of the jet varies. This shortcoming is illustrated in FIG. 2, where a cell 3 is surrounded by its insulation reservoir 2 and which contains the liquid 4 to be transferred to the not shown target. At the start of cell operation, the level of liquid 4 is at A and is at B at the end of use, the molecular jet profile obtained in the space being indicated in solid line form at 9 for level A and at 10 in broken line form for level B. It is readily apparent that this profile variation is usually unacceptable for ensuring the necessary quality characteristics with respect to the homogeneity and invariability of the crystalline deposit produced on the substrate by epitaxy.